This invention relates to ultrasonic techniques for exploring internal structures and non-invasive treatment of internal structures, and more particularly to an improved transducer and lens combination for generating narrow cross-section ultrasound beams for use in pulse-echo imaging and ultrasonic neurosurgical systems.
In recent years considerable progress has been made in the utilization of ultrasonic techniques for the exploration of the internal structure of living organisms. This technique has been used to measure and record the dimensions and position of deep-lying organs, and physiological structures throughout the body. An important advantage of ultrasonics is that it is non-destructive at power levels for pulse-echo imaging and free of the hazards incident to the use of x-ray or gamma ray examination, yet useful at higher ultrasound power levels focussed at a desired point within the body (at the location of a tumor, for example) in "burning out" and destroying the tumor without serious damage to surrounding tissue.
In the exploratory or pulse-echo imaging technique, a series of very short ultrasonic pulses is projected in a narrow, straight beam in the direction to be viewed. For this purpose, a transducer is coupled to the skin with a cream or fluid, and measurements are based on the amount of time for an echo to return to the transducer and also on the amplitude of the echo. Such echoes are produced from biologic structures which present a different acoustic impedance to the traveling pulses. Interfaces reflect not only if they are of different density, but also if they are of different elasticity.
A variety of procedures are known for producing patterns on a cathode ray viewing screen representative of the internal structure being scanned. Echo pulses may be displayed on an "A" cathode-ray indicator, the echo pulses from the different reflecting targets being displayed as "pips" of varying height along the time base sweep line on a screen. The height of each pip is indicative of the relative reflectivity of the target, whereas the displacement of the pip with respect to the point of origin of the sweep lines is indicative of the distance between the target and the transducer. In the known "B" type scan, the electron beam of the cathode-ray indicator is intensity-modulated by returning echoes as the transducer beam is shifted across a particular area of the body, and the electron beam is deflected in synchronism therewith. In this presentation, the view is similar to a cross-section taken at right angles to the axis of the transducer. Thus, it is possible with either of these scanning techniques to definitely determine the depth coordinate of an internal structure, such as a tumor, which exhibits different reflectivity than the surrounding tissue, the degree of definiteness of location of the internal structure being dependent on the imaging resolution of the system which, in turn, is proportional to the cross-section of the ultrasonic beam.
Once the coordinates of the internal structure are determined by pulse-echo imaging techniques, it is known that ultrasound energy precisely focussed on the internal structure can destroy the structure and produce a lesion of a predetermined volume, which, again, is proportional to the cross-section of the ultrasonic beam. For proper determination of the depth coordinates and the formation of a lesion volume sufficiently small to preclude damage to surrounding tissue, extremely narrow beams of less than 0.5mm diameter are required. This required beam size approaches the wavelength of the transmitted ultrasound waves, and, accordingly, unless a shaped transducer or a lens is used, diffraction effects limit the minimum beam size to about 5 to 10mm. Moreover, it is desirable that the transducer have as large an aperture as possible to maximize the capture angle of the reflected echoes and/or to maximize the ultrasound power output. However, it is virtually impossible in the current state of the art of piezoceramic transducers to construct a large aperture transducer of proper shape to produce an ultrasonic beam of the required small diameter, and if construction of such shaped transducers were possible, the cost would be prohibitive. Available machining and polishing techniques make it much easier to fabricate flat, disc-shaped, large aperture piezoceramic transducers, and attempt to employ a lens in conjunction therewith to achieve the desired beam cross-section.
Earlier studies reported in the literature have investigated the focusing properties of spherical lenses with the objective of keeping the aberrations to a minimum over large angular fields-of-view of the lens, which has entailed the design of doublet, triplet and other multi-element lens combinations using lens materials of various refractive indices. Although such lens combinations improve the quality of off-axis imaging, the designs are very expensive to implement, and moreover, when used in conventional reflective ultrasound imaging, the several interfaces of the multi-element lens lead to multiple reflections which cause artifacts in the image. From what has been said earlier, in known ultrasonic imaging and non-invasive surgical systems, the need is for extremely narrow on-axis beams, and it matters little what happens to the quality of the ultrasound radiation off-axis. The present invention provides the necessary small beam cross-section by the use of an ellipsoidalshaped lens in conjunction with a flat, disk-shaped transducer.
In the optical domain, the use of elliptical geometry to obtain aberration-free on-axis focusing was described by Descartes in 1637, a few years after Snell had stated the Law of Refraction; however, such lenses could not be tested, even in optical systems, due to the complexities in machining an ellipsoidal surface with the necessary tolerances of less than one quarter of the wavelength of light. The wavelengths generally associated with diagnostic ultrasound are in the range of 0.05 to 1.5mm. Employing state-of-the-art conventional machining processes it is possible to construct ellipsoidal lenses with surface tolerances much less than the wavelength of ultrasound. This being the case, and in view of the requirements of sharp on-axis focussing in pulse-echo imaging and non-invasive surgery systems, it is found that the ellipsoidal lens is best suited for such applications.